1. Field of the Invention
The present invention relates to the analyzing of an object to determine a composition of a layer of the object. More particularly, the present invention relates to a method of and apparatus for analyzing ions in a layer of an object using secondary ion mass spectrometry.
2. Description of the Related Art
Semiconductor devices having a high degree of integration and which operate reliably at a high speed have been developed to process massive amounts of data in a short amount of time. In general, such highly integrated semiconductor devices are fabricated by performing various processes on a semiconductor substrate. These processes include, for example, a process of forming a layer on the substrate, an ion implantation process, an etching process, and a process for forming a wiring pattern on the substrate. Ultimately, though, the semiconductor devices must be analyzed to detect for processing failures or the like.
One apparatus for analyzing an object, such as a semiconductor device, is a secondary ion mass spectrometer (SIMS). The SIMS irradiates first ions having a kinetic energy of about 0.5 keV to about 20 keV onto a surface of the object such as a semiconductor device to break the bonds between the atoms or molecules of a material that makes up the surface of the object. The breaking of the bonds by the first ions is referred to as a sputtering. Thus, it can be said that the material is broken down into elementary particles by an elementary or a molecular unit by the sputtering. These particles are partially ionized to create secondary ions. The secondary ions are then supplied to an ion analyzer. The ion analyzer classifies the secondary ions according to their energy or mass to determine the composition of the material.
A SIMS may be used to perform a mass spectrum inspection, a depth profiling inspection, an ion imaging inspection and a quantification inspection of an object. The mass spectrum inspection determines the numbers of different types of secondary ions according to the ratio of the mass of each secondary ion to the electric charge of the secondary ion, to thereby provide a representation of the kinds of atoms in and composition of the surface of the object. The numbers of the secondary ions are counted at a rate of above about 1 count/sec. to about 109 counts/sec. Accordingly, the numbers of the secondary ions are provided on a logarithmic scale. Also, the masses of the secondary ions detected using the mass spectrum inspection method are about 1 atomic mass unit (amu) to about 300 amu. Accordingly, the mass spectrum inspection method can also be used to determine the molecules, particle clusters and isotopes making up the material of the object.
The depth profiling inspection discriminates particular ions from among the secondary ions. Intensity levels of the selected ions are measured over time to determine a distribution of the selected ions over the depth of the object.
The ion imaging inspection correlates the secondary ions to the positions at which they were generated, and measures the intensities of the secondary ions. For example, the ion imaging inspection scans the object with ions having a diameter of about 1 μm, and measure intensities of the secondary ions produced during the scan. The ion imaging inspection thus determines the distribution of the secondary ions across the surface of the object.
The quantification inspection simultaneously analyzes the object and a standard object under same conditions. The standard object is fabricated by an ion implantation process so that it has ions in a concentration and at a depth which are known. A comparison between the standard object and the object being analyzed is used to obtain the concentration and depth of particles making up the object under analysis.
When used to carry out the inspection methods described above, a SIMS has a good deal of sensitivity and a wide detection range. That is, a SIMS can be used to detect infinitesimally small particles in amounts of parts per million to parts per billion. Additionally, a SIMS can detect every element in the Periodic Table, and even the isotopes thereof.
In addition, a SIMS has the ability to perform a depth profiling of about 4 nm and a line scanning of about 200 nm. Accordingly, a SIMS is useful for detecting impurities in a semiconductor device. However, certain phenomena such as a surface effect, an interface effect, an electric charge effect, and a mass interference effect, can limit the effectiveness of the SIMS when applied to detecting impurities in a semiconductor device.
The surface effect is a phenomenon in which the profile of the surface due to first ions are implanted into the surface of the object and into contaminants on the surface of the object.
The profile of the surface displayed as a result differs from that actually exhibited by the surface. The interface effect is a phenomenon in which the locations of an interface in the object are inaccurately displayed due to a number of factors such as the surface roughness of the object, variations in the first ions, and contaminants.
The electric charge effect is a phenomenon in which charges accumulate on the surface of the object due to a collision of the first ions, and the generation of the secondary ions. The mass interference effect is a phenomenon in which a single atomic ion interferes with a molecular ion and a multiple charged ion.
It has thus been proposed to rotate the object during its analysis as a way to lessen the effects of the above-described phenomena on the inspection method carried out by a SIMS. When a rotating object is irradiated with the first ions, the sputtering rates of the atoms in the object vary so that minute impurities contained in the object can be detected. Accordingly, a composition of the object can be precisely analyzed.
FIG. 1 is a graph showing an analysis of a semiconductor device using a conventional analyzing method. FIG. 2 is a graph showing an analysis of a semiconductor device using a conventional rotation analyzing method in which the semiconductor device is rotated. The semiconductor device includes a silicon layer, a titanium layer formed on the silicon layer, and a gold layer formed on the titanium layer.
A surface of the semiconductor device was irradiated with primary ions. The primary ions etched the surface of the semiconductor device. Thus, the particles detected over time corresponded to the layers of the semiconductor device in a depth-wise direction of the semiconductor device, respectively. Particles of gold, titanium and silicon layer were detected over time, as shown in FIGS. 1 and 2.
However, the interfaces of the layers were detected imprecisely in the results of the method shown in FIG. 1. In particular, the gold layer was detected in a range A and the titanium layer was detected in a range B that overlapped range A to a great extent. Thus, the gold layer infiltrated the titanium layer. Also, the results showed that the titanium layer infiltrated the silicon layer.
On the contrary, in the method shown in FIG. 2, after about 900 seconds, that is a range A′, the gold layer was not detected. After about 1,100 seconds, that is in a range B′, the titanium layer was not detected. Thus, as these results show, the method of analyzing a rotating object can determine the composition of a semiconductor device with better accuracy than a method of analyzing the semiconductor device while it is stationary.
However, analyzing a plurality of objects using a conventional SIMS is difficult. When a single object having a diameter of about 80 mm is disposed on a stage in an ion chamber of the SIMS, it is relatively easy to rotate the object by rotating the stage about its center. However, it becomes more difficult to control the rotation of the stage the larger the stage becomes. Therefore, when a large number of objects are disposed on a stage in the chamber of the SIMS, rotating the objects is problematic because the stage must be large enough to accommodate all of the objects. Furthermore, the object under analysis on the rotating stage might not be accurately irradiated with the primary ions. As a result, although the object is analyzed while it is rotated, the accuracy of the analysis can be expected to be poor.
Furthermore, various conditions are created in the ion chamber to facilitate the analysis of an object. Therefore, when objects are placed one-by-one in the ion chamber and analyzed, the atmosphere within the ion analysis chamber must be checked and often adjusted each time. Accordingly, a large amount of time is required for analyzing the objects which adds which to the overall costs associated with the analyzing process.